Determining the distance from an operation microscope to an object is very important, especially in surgery—for example in neurosurgical applications, since it is common for operations to be carried out with navigation systems and for the microscope to be used as a non-contact sensor. Once focusing has been carried out successfully via the defined measurement integral, for example, over a limited area around the focal point, a measured value can be sent to the navigation system, processed there and subsequently indexed in the data record of the object, which is generally the patient. In the currently known focusing systems, the working distance which is set once the image sharpness has been optimized is used for determining the working distance between a stereo microscope and the object surface. The evaluation of the image sharpness may be carried out in the entire field of view or in a restricted field of view, for example in a narrowly defined region around the centre of the field of view, normally a substantially point-like region with a diameter of approximately 1 to 2 mm. The focusing is normally carried out via the primary objective.